REFLECTIVITY MEASUREMENTS OF FEMTOSECOND CARRIER AND FIELD-DYNAMICS IN SEMICONDUCTORS

Carrier and field dynamics in a photoconductive switch are investigate d by voltage modulated reflectivity. Measurements are performed with s ingle, femtosecond pulsed and cw probe beams, and with pulsed pump and probe beams. We investigate the spatial dependence of the reflectivit y changes and show that for mode-locked probe light the signal at the positive electrode completely dominates the response. The comparison o f above and below bandgap excitation and reflectivity changes resolves the contributions of carrier-induced field changes and those due to t he modulation of the applied field. Photoconductive switches on p- and n-doped substrates show that field enhancement at the Schottky barrie rs has only secondary influence. We explain the results in terms of a two-dimensional field distribution which produces the largest field gr adient and hence acceleration of carriers at the electrodes and that t he higher mobility of electrons accounts for the dominant signal appea ring at the positive electrode.